Various electrostatic separators have been used for separating solid particles from gas streams. Often, the electrostatic separators are two-staged systems which include a pre-charger where the particles in the gas stream are charged, and spaced electrodes between which an electric field is created such that the charged particles are separated from the gas stream and are precipitated on a collecting electrode.
In plate-type separators these electrodes are made as plates that provide a well-developed collecting surface.
Disadvantageously, the conventional plate-type electrostatic separators have certain drawbacks, which include collection efficiency reduction due to high or low dust resistivity, reentrainment due to mixing of gas and broken dust layer, leakage of untreated dust from sides of the electrodes, and sweepage due to leakage from below the electrodes over the hoppers. When the dust resistivity is great enough, the potential gradient through the dust layer formed on the collecting electrodes may locally exceed the layer's breakdown potential. This causes a phenomenon known as "back-corona", "back-discharge", "back-ionization", or "reverse-ionization" and reentrainment of collected particles in the clean stream. On the other hand, when the resistivity of the dust is low, there is little force to hold it on the collecting electrodes. Not only is the dust held insecurely, but it packs together loosely so that its cohesivity is also low. Therefore, the dust can be removed from the electrodes by high gas velocities.
Rapping reentrainment in severe cases can account for more than 90% of the outlet dust burden. When rapped, poorly cohesive dust tends to break into a cloud of small clumps instead of falling neatly into the hopper as a coherent sheet. As a consequence, much of the dust returns to the gas flow and, unless it is intercepted, will escape from the precipitator outlet, thereby lowering collection efficiency.
Certain attempts have been undertaken in the art for improving the collection efficiency of the existing plate-type electrostatic separators. Specifically, the prior art discloses gas-permeable discharge and grounded electrodes forming a section wherein pre-charged particulates are separated from the gas stream and collected on grounded electrode surfaces. Different configurations of these electrodes, including planar, circular, v-shaped, etc., are suggested, as well as means for pre-charging particulates. These systems are described in numerous patents and publications.
For instance, U.S. Pat. Nos. 2,142,128 and 2,142,129 disclose an electrical precipitator in which a gas stream passes first through the ionizing field and then moves towards two perforated electrodes crossing the gas stream.
The first of these perforated electrodes is charged at the same polarity as the particles, such that the particles are repelled from this electrode towards the grounded collecting electrode whereon they are precipitated. A satisfactory efficiency is intended due to the gas stream and the precipitating field exerting their respective forces in the same direction on the suspended particles, thereby reinforcing each other in effecting precipitation of the particles.
The collecting effectiveness of electrostatic precipitators employing gas-permeable grounded collector plates situated downstream of the discharge electrodes can also be increased when a filter media is disposed between the electrodes. In this case, an electrical field exists through the filter media and the particles leaded by this electrical field are retained in the filter media. These electrostatic filters are disclosed in U.S. Pat. Nos. 2,729,302; 3,910,779; 3,915,676; 3,966,435; 3,999,964; 4,205,969; 4,354,888; 4,357,151; 4,405,342; 5,403,383; and 5,474,599. The filters are periodically removed and cleaned or discarded.
When gas-permeable grounded collector plates are situated downstream of the discharge electrodes, and electrostatic forces act in the same direction as drag forces, the dust layer can be formed and held securely on the collecting electrode surfaces, especially if a filter media is disposed between the electrodes. In these cases, the collectors are able to provide high collection efficiencies, but clean stream should penetrate through pores of the dust layer, and collectors will have relatively high pressure drops in comparison with conventional plate-type electrostatic precipitators at comparable conditions. If the dust layer on the electrode surfaces has not been formed yet (i.e., after cleaning the collector surfaces), the collector efficiencies are relatively low (similar phenomena are observed for bag filters after their cleaning cycles).
The prior art discloses also electrostatic precipitators with the gas-permeable grounded electrodes situated upstream of the discharge electrodes. For instance, U.S. Pat. No. 3,616,606 discloses a multistage electrostatic precipitator which includes a first, or conventional, pre-charging section and a second section comprising a plurality of parallel perforated plates traversing the gas flow. The first grid of the second section is charged to a positive potential, and the remaining grids are arranged such that each two adjacent plates are oppositely charged. Once the negative particles which are not detained in the first section enter the second section, they are attracted by the first grid and are collected thereon. Those which have not been affected by the first grid, pass through the second, negatively charged, grid and are collected on the third grid, positively charged, etc. Some of the negatively charged particles passing through the second grid will be repelled therefrom and return to the first grid, where they will be collected and removed. Similarly, the positively charged particles will be collected on the negatively charged grids.
U.S. Pat. No. 3,668,836 discloses an electrostatic precipitator with respective grounded collector plates upstream of the adjacent electrically charged wires. The first perforated plate is transversely disposed in the duct, so that the gas stream initially passes through the openings in the first plate. A high voltage potential is maintained between the ionizing wires and the grounded plates, and the entrained discrete particles are deposited from the gas stream onto the plates, due to an electrostatic precipitation mechanism in which the particles receive a charge from the wires and are discharged by and onto the plates.
Like other types of electrostatic precipitators with gas-permeable electrodes, the systems disclosed in U.S. Pat. Nos. 3,616,606 and 3,668,836 require collecting electrodes which should be able to hold securely the dust on the collecting electrode surface. Inability to hold this dust results in reentrainment of particles in the clean stream. However, when grounded collector plates are situated upstream of the discharge electrodes, and electrostatic and drag forces act in the opposite directions (i.e., U.S. Pat. Nos. 3,616,606; 3,668,836), drag forces promote removing particles from the collecting electrode surfaces (especially particles covering the plate apertures) and reentrainment of these particles in the clean stream.
As can be seen, a necessary prerequisite required to achieve high collection efficiencies for all prior art systems, including those employing gas-permeable discharge and grounded collecting electrodes, is the collecting electrode ability to hold securely the dust on the collecting electrode surface. In some cases, i.e., when a filter media is disposed between electrodes, the dust layer can be formed and held securely. However, these systems have relatively high pressure drops. When the dust layer cannot be formed or held securely on the collecting electrode surfaces, the electrostatic precipitators will have relatively low collection efficiencies.
It will be greatly advantageous to design an electrostatic separator which would be able to employ gas-permeable discharge and grounded electrodes but which collection efficiency would not depend on the system ability to form and hold the dust layer on the grounded electrode surfaces. This separator would not have the shortcomings and deficiencies of the existing state-of-the-art electrostatic precipitators.